1) Field of the Invention
The present invention relates to micro electro mechanical system (MEMS) micromirror, and specifically, to a micromirror element suitable for a large scale channel switching (that is, crossconnect) for wavelength multiplexed optical signals with a large number of wavelength multiplex, and to an optical switch with the use thereof.
2) Description of the Related Art
Recently, a necessity for handling optical signals at an ultra high speed exceeding 10 Gbp per second has been arising in an optical switch function of an optical cross-connect device or the like as a consequence of speedup of optical signals in trunk line system. Moreover, the optical switches are becoming larger in size owing to an increase in number of wavelength multiplex in the transmitting technology of Wavelength Division Multiplex (WDM).
Large scale optical switches with MEMS mirrors have been disclosed in Patent literature 1 and Non-Patent literature 1. Hereinafter, the MEMS mirror used for an optical switch in such a structure is explained.
In the MEMS mirror element, a pair of electrodes arranged on a substrate carry out attraction and separation of a mirror swayably supported above the substrate by an electrostatic force generated between the electrodes. Owing to the electrostatic force, the mirror sways and tilts, thereby allowing adjustment of angles of the reflecting surface of the mirror. Moreover, owing to appropriate adjustment of the angles of the reflection surface of the mirror, it is possible to carry out switching of signal paths of light (for example, see Patent literature 2).
In the MEMS mirror element in such a structure, for example, a pair of electrodes are formed on a substrate by etching, and also a mirror body is constituted of a mirror and a mirror frame that swayable supports the mirror. Moreover, the mirror body is arranged via a support on the substrate on which a pair of electrodes are formed.
In an optical switch provided with such a MEMS mirror element described above, it is required to carry out feedback control using input light and output light for the optical switch to realize a stable operation. In addition, it is also necessary to compare the input light to the output light to judge malfunction inside the switch. Therefore, an arrangement of light detection devices in an input unit and an output unit of light, respectively, in the optical switch is necessary.
FIG. 15 represents a conventional optical switch provided with MEMS mirror elements. As shown in FIG. 15, in an optical switch provided with MEMS mirror arrays for input side 1505 and output side 1506 having a structure in which a plurality of MEMS mirrors 1500 are arranged in a matrix form and integrated, light (input light) 1511 input to an optical fiber array for input side 1503 is detected by a light detection device for input side 1501, and light (output light) 1512 output from an optical fiber array for output side 1504 is detected by a light detection device for output side 1502. A control device 1507 compares light detection signals 1513 and light detection signals 1514 that are obtained from the input light and the output light detected by the light detection devices for input side 1501 and output side 1502, respectively, and detects malfunction of the optical switch based on the comparison results. Furthermore, feedback control of the optical switch is carried out based on the detection signals for output light 1514. Each of the light detection devices for input side 1501 and output side 1502 makes use of the one that integrates, for example, an optical coupler and an optical monitor.
As to the optical switch having the structure described above, the light detection devices 1501 and 1502 are arranged on the input side and the output side of the optical switch, respectively, which makes the structure of the switch complex, resulting in a significant increase in cost. To solve the problem, a structure shown in FIG. 16 in which light detection devices are integrated on a mirror surface has been offered (for example, see Patent literature 3).
In the MEMS mirror element shown in FIG. 16, a mirror body 1600 is arranged above a substrate (not shown) via a support (not shown). In the mirror body 1600, a light detection layer 1603 is formed on the surface of the mirror substrate (not shown), and a reflecting layer 1604 constituting a mirror surface (reflecting surface) is formed on the surface of the light detection layer 1603 to constitute a movable mirror 1601. The movable mirror 1601 is swayably supported by a first mirror frame 1602 via torsion springs 1605, and the first mirror frame 1602 is further swayably supported by a second mirror frame 1606 via other torsion springs 1605.
In the MEMS mirror element in such a structure, a light current is detected on the light detection layer 1603 of the movable mirror 1601 according to input light, thereby obtaining detection signals of the light incident to the MEMS mirror element. Since the movable mirror 1601 and the light detection layer 1603 are integrally formed in the MEMS mirror element, separate light detection devices are not necessary as in the case of FIG. 15.    Patent literature 1: International Publication WO 00/020899    Patent literature 2: U.S. Pat. No. 6,044,705    Patent literature 3: Japanese Patent Application Laid-Open Publication No. 2003-202418    Non-Patent literature 1: Fully provisioned 112×112 micro-mechanical opticalcrossconnect with 35.8 Tb/s demonstrated capacity, Optical Fiber Communications Conference (OFC 2000), Postdeadline paper PD-12, March 2000
In the MEMS mirror element of the structure shown in FIG. 16 in which the movable mirror 1601 and the light detection layer 1603 are integrally provided, the yield rate of the movable mirror 1601 and the yield rate of the light detection layer 1603 synergistically affect the yield rate of the MEMS mirror element at the time of fabrication of the element. Accordingly, the yield rate of the whole MEMS mirror element is reduced in such a structure. As the result, reduction in cost becomes difficult.